Cohesive coating compositions

ABSTRACT

A cohesive coating composition comprises in dispersion in a liquid a mixture of between 51 and 99% by weight of a natural or synthetic rubber and between 1 and 49% by weight of a polyalkyl methacrylate in which dispersion there is incorporated between 5% and 35% by weight, based on the weight of the solids, of an inert particulate material having a mean particle size lying in the range between 0.2 and 50 microns. 
     The compositions are applied to sheet materials used, for example, in packaging.

This invention relates to cohesive coating compositions, that is,coating compositions which when coated on one side of a suitable sheetmaterial such as flexible films or paper, permit the sheet materials tobe firmly sealed together, coated side to coated side, under pressure atambient temperature but do not seal to any significant degree underpressure when the sheet materials are placed coated side to non-coatedside.

Such cohesive compositions are well-known and are particularly useful inpackaging applications where conventional heat-sealing gives rise todifficulties, for example in the packaging of chocolate or icedconfections. The compositions are usually based upon a natural orsynthetic rubber and it is important in practice that the compositionadheres strongly to its base sheet material to ensure that strong sealsare obtained. It is also important that there is no blocking between theuncoated and coated sides of the sheet material when it is wound in rollform otherwise smooth unwinding on packaging machinery is adverselyaffected and, when the sheet material is surface printed, ink transfermay occur.

The present invention is concerned with cohesive coating compositionshaving improved properties particularly in respect of antiblocking andanchorage to base sheet materials.

According to the present invention there is provided a cohesive coatingcomposition comprising in dispersion in a liquid a mixture of between 51and 99% by weight of a natural or synthetic rubber and between 1 and 49%by weight of a polyalkyl methacrylate in which dispersion there isincorporated between 5% and 35% by weight, based on the weight of thesolids, of an inert particulate material having a mean particle sizelying in the range between 0.2 and 50 microns.

In a more preferred form of the invention the natural or syntheticrubber is present in the mixture in the range between 70 and 90% byweight and the polyalkyl methacrylate in the range between 10 and 30% byweight.

Preferably a suitable stabiliser and a conventional antioxidant andpreservative are also added to the mixture. The synthetic rubber may,for example, be a carboxylated styrene butadiene rubber.

The inert particulate material preferably is present in the rangebetween 10% and 25% by weight based on the solids in the dispersion. Itmay be a plastics material such as polyvinyl chloride but preferably itis of a mineral origin, for example glass spheres, which may be solid orhollow such as is marketed as "Ballotini" or, more preferably, hydratedaluminum silicate. If the mean particle size of the particulate materialis below about 0.2 microns then there is a tendency for blocking tooccur when the surface of a sheet material, for example a film, coatedwith the composition is pressed against an uncoated surface of thesubstrate, for example when a single side coated substrate is wound intoa roll. If, however, the mean particle size of the particulate materialis above about 50 microns there is a fall off of the cohesive propertiesof the composition. Preferably the mean particle size of the particulatematerial lies within the range between 2.0 and 20 microns.

A preferred polyalkyl methacrylate is poly n-butyl methacrylate.

The coating composition may be applied to the surface of any of theconventional sheet wrapping materials by any of the conventional coatingmethods, for example by means of gravure rollers, flexographic plates orair-knife doctoring techniques. Examples of suitable sheet wrappingmaterials are plastics films, such as high density and low densitypolyethylene and polypropylene films, cellulose film or paper. Theplastics film may be surface treated with corona discharge or otherknown oxidising treatment to render the surface receptive and moreadherent to the coating composition.

The invention also includes a method for the production of a cohesivecoated sheet wrapping material comprising coating a sheet wrappingmaterial with a cohesive coating composition in accordance with theinvention followed by drying the composition and sheet wrapping materialwhen produced by such method.

The invention will now be further illustrated by the following Examples:

EXAMPLE 1

350 grams of a natural rubber latex containing 60% by weight solids andincorporating a conventional stabiliser and antioxidant and preservative(marketed by Revertex Limited of Harlow, Essex as "Revertex B 1850/1")was mixed with 75 grams of a high pH stable poly n-butyl methacrylatelatex containing 50% by weight solids (marketed as "VINACRYL 4290" byVinyl Products Limited, Carshalton, Surrey), 50 grams of a hydratedaluminum silicate having a particle size predominantly in the rangebetween 2 and 10 microns and an average particle size of 4.5 microns(marketed as "HYDRITE Flat D" by the Georgia Kaolin Company Ltd., NewJersey, United States of America) and 240 grams of deionised water. Themixture was stirred gently with care being taken not to coagulate thesolids.

The mixture was then coated on one side of a vinylidene chloridecopolymer coated cellulose film and the coating was doctored with anair-knife to provide after drying a solid coating having a weight of 2.5grams per square meter. The drying was effected by infra red heat.

The seal strength of the coated film was determined by cutting samplestrips 11/2 inches in width, pressing a portion of two such alignedstrips together, coated side to coated side, between a pair of sealingjaws at room temperature and then determining the force required to peelthe strips apart at the seal.

On using a pressure of 40 pounds per square inch for one second betweenflat sealing jaws the average seal strength taken from five sealedstrips was 220 grams per 11/2 inches width. On using a pressure of 64pounds per square inch for one second between crimp sealing jaws theaverage seal strength was found to be 280 grams per 11/2 inches width.

On repeating the seal strength measurements on strips seven days aftercoating the seal strengths were found to be 280 grams per 11/2 incheswidth for flat jaw sealing and 350 grams per 11/2 inches width for crimpjaw sealing.

Further, a roll of the coated film which had been stored for four weekswas found to unwind with substantially no sticking.

EXAMPLE 2

The following ingredients were added to a mixing vessel one at a time inthe listed order and thoroughly mixed by gentle stirring before theaddition of the next ingredient.

    ______________________________________                                        Parts solids by weight                                                        ______________________________________                                        Natural rubber latex stabilised                                               with ammonium caseinate and including                                         butylated hydroxy toluene (anti-                                              oxidant) and 1:2 benzisothiazolone                                            (preservative)               58                                               Ethylene oxide fatty alcohol                                                  condensate (stabiliser) marketed                                              as "VULCASTAB-LW" by Imperial                                                 Chemical Industries Ltd.     0.1                                              Poly n-butyl methacrylate latex                                               containing 10% dibutyl phthalate                                              (plasticiser) -, "VINACRYL 4290"                                                                           16                                               Hydrated aluminum silicate                                                    particles of size in the range                                                between 2 and 10 microns -                                                    "HYDRITE FLAT D"             26                                               ______________________________________                                    

Sufficient de-ionised water was then added to provide a dispersioncontaining 53% solids by weight.

The dispersion was coated on to one side of a vinylidene chloridecopolymer coated cellulose film by means of a rotatable engraved rollerdipping into a bath of the dispersion. The roller was doctored with adoctor blade placed at an angle reverse to that which is conventionalwhereby the edge of the blade was set in opposition to the direction oftravel of the roller. The coating was applied at a rate to provide 5.6grams dry solids per square meter and drying of the coating was effectedby hot air.

The seal strength of the coated film was determined in the mannerdescribed in Example 1 after 24 hours, after seven days and after threeweeks using flat sealing jaws at a pressure of 40 pounds per square inchfor one second and crimp sealing jaws at a pressure of 64 pounds persquare inch for 0.8 seconds.

The seal strengths obtained in grams per 11/2 inches width were asfollows:

                 Flat jaws     Crimp jaws                                         ______________________________________                                        24 hours        70             125                                             7 days        320             260                                             3 weeks       460             290                                            ______________________________________                                    

It was also found that a roll of the coated film which had been storedfor four weeks was found to unwind with substantially no blocking.Further, chocolates sealed in pouches of film coated with the coatingcomposition showed no sign of odour or taint after three months.

EXAMPLES 3 to 8

The cohesive composition as prepared in accordance with Example 2 wascoated on one side of six different sheet materials and was doctored byhand using a K-bar to provide after drying a solid coating having aweight of 3.0 grams per square meter.

The seal strengths of the coated materials were determined after 24hours, 7 days and 3 weeks using flat seal jaws and crimp seal jaws underthe same conditions as described in Example 2 with the results set outin the following Table.

                                      TABLE                                       __________________________________________________________________________    Example No.                                                                             Sheet material                                                                          Seal strength in grams/11/2 inches width                                      24 hrs. 7 days  3 weeks                                   __________________________________________________________________________    3.      Low density poly-                                                                         Flat                                                                              Crimp                                                                             Flat                                                                              Crimp                                                                             Flat                                                                              Crimp                                         ethylene (corona                                                              discharge treated)                                                                        280 320 330 360 390 300                                   4.      High density poly-                                                            ethylene (corona                                                              discharge treated)                                                                        330 400 300 390 410 410                                   5.      Biaxially orientated                                                          polypropylene                                                                 (corona discharge                                                             treated)    475 460 390 420 300 360                                   6.      Vegetable parchment                                                           paper.      450 695 470 700 440 1020                                  7.      Aluminium foil                                                                            530 770 540 680 570 700                                   8.      Ethylene/propylene                                                            copolymer coated poly-                                                        propylene   300 370 390 510 570 430                                   __________________________________________________________________________

I claim:
 1. A cohesive coating composition comprising in dispersion in aliquid a mixture of between 70 and 90% by weight of carboxylated styrenebutadiene rubber and between 10 and 30% by weight of poly-n-butylmethacrylate in which dispersion there is incorporated between 10% and25% by weight, based on the weight of the solids, of particulatehydrated aluminum silicate having a mean particle size lying in therange between 2.0 and 20 microns.